Primary or secondary type recovery methods that are well-known to specialists can be used in order to better displace petroleum fluids towards production wells. The recovery is referred to as primary when the in-situ energy is used. Expansion of the fluids that are initially under high pressure in the reservoir allows part of the oil in place to be recovered. During this stage, the pressure in the reservoir can fall below the bubble point and a gas phase appears, which contributes to increasing the recovery ratio.
Secondary type recovery methods are rather used in order to avoid too great a pressure decrease in the reservoir. The principle consists in displacing the petroleum fluids by means of an energy supply external to the reservoir. Fluids are injected into the reservoir through one or more injection wells and the petroleum fluids displaced (referred to as "oil" hereafter) are recovered by means of production wells.
Water can be used as a displacement fluid but it has a limited efficiency. A large part of the oil remains in place notably because the viscosity thereof is often much higher than that of water. Besides, the oil often remains trapped by the contractions of the pores due to the great interfacial tension between the oil and the water. Since the reservoir is often heterogeneous, the water readily sweeps the most permeable zones while bypassing the others, hence a great recovery loss.
It is also well-known to inject pressurized gas that penetrates the pores of the rock and displaces a large amount of the oil in place. Even if water has first been injected into the reservoir, as it is often the case, gas has a well-known property of displacing an additional amount of oil that can be significant.
A notable drawback of this recovery technique using gas is that the latter is much less viscous than the oil to be displaced and than the water possibly in place. Because of the high mobility thereof, the gas flows through the reservoir by following only some of the most permeable channels that reach the production well(s) without displacing a large amount of oil.
If the reservoir is not homogeneous and comprises layers or cores of different permeability, this effect becomes still more pronounced and the gas, bypassing the least permeable zones, reaches the production wells even faster. When the gas thus breaks through prematurely without having the expected displacement effect, it loses all of its efficiency. To continue injection thus has no practical effect any more.
It is also well-known to combine the two techniques according to a method referred to as WAG method. Water and gas are successively injected and this sequence is repeated by alternating water slugs and gas slugs as long as oil is produced under good economic conditions. This combined injection method produces better results since the mobility of the gas of each slug, which is more efficient than water at the level of the pores, is relatively reduced by the presence of the water slug preceding it. However, as a result of the reduced volume of the slugs in relation to the distance they must cover between the injection wells and the production wells and of the heterogeneity of the reservoir, the efficiency of the macroscopic sweep does not last long. Surfactants can be added to the water in order to decrease the water-oil interfacial tension and to improve the efficiency of these combined injections. The foam that forms in the presence of the gas has the effect of reducing the mobility of the gas and the fingerings. Such a method using alternate slugs is for example described in patent U.S. Pat. No. 5,465,790.
Patent FR-2,735,524 filed by the applicant describes a method allowing to displace petroleum fluids out of an underground reservoir by means of successive injections, through one or more injection wells, of slugs consisting of a wetting fluid such as water and of gas slugs, and the recovery, through one or more production wells, of the petroleum fluids displaced by the wetting fluid and the gas injected. This method mainly consists in adding to at least one slug of the wetting liquid injected an amount of substances suited to make the spreading coefficient negative. Alcohol is notably used in a proportion of 1 to 5% by weight for example. It may be, for example, a low molecular weight alcohol belonging to the isobutyl or isoamyl alcohol class. Light polar compounds such as amines, fluorinated products or light acids may also be used.